1. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to an apparatus and method for processing a General Packet Radio Service (GPRS) Tunneling Protocol (GTP) for lossless and sequential transmission of packet data on a user plane for a handover of a wireless mobile terminal (hereinafter referred to as a user terminal) in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system.
2. Description of the Related Art
Unlike the Universal Mobile Telecommunications System (UMTS) of the related art, the 3GPP LTE system does not include a Radio Network Controller (RNC) and has a simple network structure including a user terminal, an evolved Node B (eNB) (hereafter referred to as a base station), and an Evolved Packet Core (EPC).
According to the simplified network structure, the GTP used for transmission and reception on a user plane between the core network device (e.g., Mobile Switching Center (MSC)) and a Radio Network Subsystem (RNS) of the 3GPP LTE system is handed down to a base station, so that the base station and the EPC interface through the GTP.
The LTE system provides a packet data service that transmits voice through the Voice over Internet Protocol (VoIP) without dividing transmission/reception processing of voice and data.
FIG. 1 is a diagram illustrating a structure of an LTE system according to the related art.
Referring to FIG. 1, in the LTE system, user terminals 110 and 120 are allocated unique IP addresses from a Mobility Management Entity (MME) of an EPC 160 to receive services. Each of the terminals 110 and 120 provides VoIP, VOD, FTP and Web browsing services to a user.
A Radio Bearer (RB) is allocated for each service between the user terminals 110 and 120 and base stations 130 and 140, and GTP Tunnel IDentifiers (IDs) are allocated respectively between the EPC 160 and the base stations 130 and 140. The EPC 160 may manage the user terminals 110 and 120 through other nodes 150, 170, 180 and 190, and may be connected to another network through a Packet Data Network (PDN) 195.
In the LTE system, a user terminal sets a default bearer through an attach process with an access network in an initial power-on mode. The user terminal sets each dedicated bearer whenever triggering each service.
A Dedicated Radio Bearer (DRB) is allocated through a Radio Resource Control (RRC) connection process between the user terminal and the base station, and a Tunnel Endpoint ID (TEID) is allocated through a context setup process between the base station and the MME to set an S1-U bearer.
Thereafter, an SAE bearer (i.e., a user traffic path) is set between the user terminal and the EPC 160 to communicate user data between the user terminal and the EPC 160. In the case of uplink data, the base station deciphers and decompresses user data, received through PHYsical (PHY), Media Access Control (MAC) and Radio Link Control (RLC) layers, by a Packet Data Control Protocol (PDCP) and transfers the resulting data to the GTP that is an upper layer.
The GTP encodes and transmits a GTP-U header including a TEID allocated at the call setup for a received Packet Data Unit (PDU). Herein, the TEID represents a destination TEID that is mapped using a traffic handling table generated at the call setup and call information (e.g., Radio Bearer ID (RBID) and Direction Indicator) received from the PDCP.
In the case of downlink transmission from the EPC, a TEID is extracted from a received GTP-U PDU, RBID and other information for processing in the base station are acquired from the traffic handing table, and an internal message is generated and transferred to the PDCP layer (the base station manages the mapping structure between the RBID and the TEID).
The PDCP layer allocates a PDCP Sequence Number (SN) for the PDU received from the GTP, compresses/ciphers an IP header in the PDU, and transfers the results to the user terminal through the RLC/MAC/PHY layer.
In a basic process for a handover between representative base stations in the LTE system, a source base station receives a measurement report message from the user terminal to determine whether to perform a handover.
In the case of RLC Acknowledged Mode (AM), if it is determined that preparation for a handover to a target base station ends, a source base station starts data forwarding and transfers a PDCP SN, allocated and used in the PDCP layer of a base station, through an SN Status Transfer message. The first PDCP SN failing to be received from the user terminal is transferred in the case of uplink transmission, and the next SN of the last allocated PDCP SN in the PDCP layer is transferred in the case of downlink transmission. The target base station buffers packets received from the source base station.
If it is determined that the handover to the target base station is completed, the target base station transfers a path switch request to the EPC so that the EPC transfers downlink user data to the target base station without transmitting the same to the source base station.
When receiving all the packet data forwarded from the source base station, the PDCP layer of the target base station re-orders the buffered packets by the PDCP SN and transmits results to the user terminal. Also, data newly received from the EPC are allocated a PDCP SN with reference to the PDCP SN of SN Status Transfer, and are received from the source base station, prior to transmission to the user terminal.
In a switch downlink path process during the handover process, the EPC should inform that there is no more transmission of user data to the source base station. Also, the source base station should inform of the completion of data forwarding to the target base station.
If the EPC does not inform the source base station that there is no more transmission of user data and the source base station does not inform the target base station of the completion of data forwarding, the PDCP layer of the source base station endlessly waits for packets from the EPC and the PDCP layer of the target base station endlessly waits for packets from the source base station.
That is, for a handover, the PDCP layer of the base station processes data to be forwarded and should know the last forward data. If not, the PDCP layer waits for data reception endlessly or waits for data reception during a timer operation period, which may cause a loss or delay of user data.
Therefore, a need exists for an apparatus and method for efficient traffic transmission between base stations and between a base station and an EPC.